This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This research is a small study being done to see if sildenafil is safe and effective in decreasing inflammation in cystic fibrosis [CF]. Lung disease in CF develops in part because of bacterial infection, and the body's response to fight the infection [called inflammation]. Some of the medications used to treat CF lung disease are aimed at decreasing this inflammation. Researchers in laboratories have shown that inflammation and infection are decreased in CF cells that are treated with sildenafil. It is not known if treating CF patients with sildenafil will also decrease inflammation and infection. The purpose of this study is to determine if treating CF patients with sildenafil decreases inflammation. We will also test whether it makes CF patients feel better and allows them to exercise more.